1. Field of the Invention
The present invention relates to a recording element substrate used in an ink jet head which performs recording by discharging ink droplets from discharging ports. The present invention further relates to a recording head having such a recording element substrate and an ink jet recording apparatus having such a recording head.
2. Description of the Related Art
A substrate for an ink jet head includes heaters (heating elements), drivers, a logic circuit, a substrate temperature sensing element, pads (external electrode terminals) and the like.
The heaters generate thermal energy for discharging ink. The drivers drive the heaters. The logic circuit controls the drivers. The substrate temperature sensing element senses a substrate temperature. The pads are used for electrical connection with the ink jet head or the ink jet recording apparatus.
The heaters are formed such that the number of the heaters is compatible with that of discharging ports. Therefore, the number of the drivers is also formed compatibly with that of the discharging ports. Such a substrate for an ink jet head is monolithically fabricated on a silicon semiconductor substrate through semiconductor device manufacturing techniques.
In addition, such an ink jet head is characterized in that temperature is closely related to a diameter and a discharging speed of an ink droplet, which affects an image density and exert an influence upon printing quality. Therefore, sensing of the substrate temperature plays an important role.
As a substrate temperature sensing element which is provided on a substrate for an ink jet head, a diode sensor and an aluminum sensor are adopted therein, each of which is formed on a silicon substrate through semiconductor manufacturing techniques.
The diode sensor senses temperature in accordance with temperature properties of forward voltage in a semiconductor diode. In addition, the aluminum sensor senses temperature in accordance with change in a resistance value caused by change in temperature.
Here, comparing the diode sensor and the aluminum sensor, the properties of the diode sensor are stable even if deviation in manufacturing processes is considered. On the other hand, in the aluminum sensor, the deviation occurs in the resistance value according to a film thickness and a line width of aluminum in manufacturing processes. For this reason, the diode sensor is excellent in view of the accuracy of the temperature sensing. In addition, in the aluminum sensor, high electrical resistance is required, and the line width has to be increases in order to reduce the deviation in processes. Therefore, such problem is dealt with by lengthening a layout distance of the aluminum sensor. The configurations proposed in U.S. Application Laid-open. No. 2002/0149657 or U.S. Pat. No. 6,945,629 have been known as a prior art for the above-mentioned techniques. In addition, U.S. Pat. No. 7,441,878 discloses a configuration such that temperature sensors are disposed at plural places of a substrate.
Sensing of a substrate temperature is important in order to enhance resolution in printing quality and an ink jet head is required to be long according to increase in printing speed. For this reason, the substrate for the ink jet head is configured to include the heaters to be arranged in a longitudinal direction of the substrate. At the same time, it is important that temperature measurement can be performed on a plurality of places including the vicinity of the center portion of the heater array as well as the temperature measurement is performed in the vicinity of end portions of the conventional heater array.
However, when the diode sensors for measuring the temperature near the end portion of the heater array are disposed not only on the end portion of the heater array but also on the center portion of the heater array to measure the temperature, the following problem occurs. That is, there is a problem in that sensitivity in the temperature measurement of the diode sensor which is disposed on the center portion of the heater array is degraded.
The causes of the problem are as follows.
FIG. 8 is a cross-sectional view schematically illustrating the diode sensor. In FIG. 8, a diode for sensing the temperature is formed to have a PN junction structure which is schematically illustrated as a diode 206. That is, in order to form the diode for sensing the temperature, a p-type region 202 and an n-type region 203 are formed on a p-type semiconductor substrate 201, and an n+ region 204 and a p+ region 205 are formed within the n-type region 203.
When such a structure of a temperature sensor is used, a constant current is supplied from the p+ region 205 serving as an anode to the n+ region 204 serving as a cathode. At this time, by monitoring a forward voltage (Vf), change in temperature can be sensed because Vf has a temperature property of about −2 to −2.5 mV/° C.
The relationship between the change in temperature and an output voltage at the time of applying the constant current to the diode sensor has the feature illustrated in FIG. 9. That is, in the diode sensor, the output voltage decreases as the temperature increases.
In contrast, the relationship between the change in temperature and the output voltage at the time of applying the constant current to an aluminum wiring for electrically connecting the diode sensor or an aluminum sensor has the feature illustrated in FIG. 10. That is, in the aluminum wiring or the aluminum sensor, the output voltage also increases as the temperature increases.
Here, in order to dispose the diode sensor in the vicinity of the center portion of the heater array, first, the diode sensor is disposed in the vicinity of the center portion of the heater. Further, it is necessary to make longer the aluminum wiring from an input/output pad to the diode sensor in order to electrically connect the input/output pad and the diode sensor.
With the configuration described above, in terms of a value of the output voltage for sensing the temperature at the time of applying the constant current, when the temperature increases, a value of the output voltage of the diode sensor unit decreases, and on the contrary, a value of the output voltage of the aluminum wiring unit increases. That is, the values of the output voltages are changed so as to be canceled to each other. For this reason, an amount of change in temperature is sensed smaller than an actual amount of change, so that the sensitivity becomes degraded.
As a result, since accuracy of the temperature sensing in a high temperature of the ink jet head is lowered, it is impossible to properly perform driving control and the printing quality is often degraded due to variation in an image density.